1. Field of the Invention
This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a micellar slug capable of forming micro-emulsions having a small interfacial tension at a high salt concentration and of recovering oil at a high oil recovery efficiency from subterranean reservoirs.
2. Description of the Prior Art
It is well-known in the art that the so-called "primary recovery" methods, including pumping methods, can recover only a portion of the petroleum or crude oil (referred to as "oil" hereinafter) from subterranean reservoirs and leave substantial amounts of oil in the subterranean reservoirs.
In order to recover the remaining large amounts of oil from the subterranean reservoirs, the so-called "secondary recovery" methods have been proposed, wherein, for example, water or gas is injected into the subterranean reservoirs from an injection well at a pressure sufficient to increase the flowability of oil, steam is injected into subterranean reservoirs so as to effect the displacement of oil toward a production well, or oil in subterranean reservoirs is partially burned to heat the subterranean reservoirs so as to decrease the viscosity of the oil and increase the flowability of the oil.
Furthermore, the so-called various "tertiary recovery" methods, including a combination of secondary recovery methods and improved secondary recovery methods, have been proposed, wherein surfactants or water-soluble polymers are utilized. These methods are generally called "enhanced oil recovery" (EOR) methods.
Among the surfactants EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, a clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates, or fuel oil, is injected under pressure into the subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These EOR methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants can be used alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonate, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
The micellar slugs should have the following characteristics to recover oil from subterranean reservoirs at a high efficiency:
Sufficiently low interfacial tensions between oil and the micro-emulsions and between formation water and the micro-emulsions; the same or slightly higher viscosity of the micro-emulsions than that of the oil contained in the subterranean reservoirs; and sufficient salinity tolerance of the micellar slugs a at a wide salt concentration since the salt concentration range of water contained in the subterranean reservoirs extends widely from a low to high concentration.